The research efforts of this laboratory center on documenting organization and variation in the nervous systems of living vertebrates and utilizing these data to elucidate mechanisms underlying brain evolution. In this application, a plan Is outlined to examine the peripheral distribution and innervation of electroreceptive ampullary organs and mechanoreceptive neuromasts, their embryonic development, and the ascending neural pathways mediating these sensory modalities of the lateral line system in a group of amphibians possessing a primitive pattern of electroreception and in a teleost fish with an independently derived pattern of electroreception. These studies will involve a number of microdissections and surgical approaches on embryos, larvae, juveniles and adults, primarily with the use of horseradish peroxidase and the indocarbocyanine dye DiI. Axolotls, Ambystoma mexicanum, a group of permanently aquatic salamanders, are one species of choice, as they represent the only group of vertebrates that retains primitive electroreceptors and whose embryology can be easily studied, and because cell lineages that give rise to the lateral line system exhibit a naturally occurring pigment-marker that facilitates donor-to-host transplantation experiments. The channel catfish, Ictalurus punctatus, provides an ideal comparison, as this teleost fish possesses secondarily derived electroreceptors, its normal development has been staged, and its embryos can be obtained in large numbers from commercial sources. The data generated by these studies will be used to evaluate a number of hypotheses regarding the developmental processes that underlie the loss and re-evolution of electroreceptors.